The invention is particularly, though not exclusively, suited to applications in the specific field of mechanical-workpiece surface machining where a balanced, centred division of the excess metal thickness removed from each of a pair of opposed surfaces is required.
A typical application is that relating to the machining of the opposed flat surfaces of a brake disc for vehicles. In this case, the surface finishing (turning, for example) requires the removal of excess metal thicknesses from these surfaces in a balanced, centred manner in relation to the brake disc's median plane of symmetry, for obvious reasons of weight balancing.
In this field, the above-mentioned operations are typically carried out by clamping the brake disc on the chuck of the machine tool by the engagement of gripping clamps of the workpiece-holder device, for example, a gripper or self-centring chuck, in the radial passages which are created in the disc during the casting of the semifinished workpiece, and which constitute the finned cooling ducts of the brake disc. Clamping the workpiece in this way defines a median reference plane relative to which the excess metal thicknesses to be removed from each of the flat disc surfaces are measured.
One of the main limitations encountered in the known applications lies in the fact that the clamping of the disc in the region of the above-mentioned median plane allows only one of the disc surfaces to be machined because the tools cannot gain access to the other disc surface, i.e. the surface facing the chuck, due to the presence of the clamping device. Once one surface of the disc has been machined, the disc must be positioned and clamped on the chuck a second time to allow the other, opposite surface to be machined.
This double positioning consequently involves an increase in cycle times with a corresponding extension of overall production times and increase in costs. A second positioning could also affect the precision of the required dimensions and tolerances, especially when it is considered that the machined disc surfaces must generally meet fairly tight tolerances of flatness and mutual parallelism.